The application of Robert E. Swenson, Ser. No. 207,152, discloses a system wherein a cache store stores segments of data, these segments containing the most recently used, or most likely to be used data. A host processor wishing to initiate a transfer of data to/from the processor from/to a disk devices issues a command to a storage control unit. The command specifies the function (Read or Write) to be performed and also specifies the disk space at which this function is to be performed. The disk space is specified by a starting address and the number of words to be transferred. The storage control unit first determines whether or not a copy of the data from the specified disk space is resident in the cache store. If it is, the storage control unit sets up a data transfer between the cache store and the host processor.
If a copy of the data from the specified disk space is not resident in the cache store then the storage control unit causes the data to be brought from the specified disk space after which the function specified by the command is carried out.
The mechanism by which the storage control unit determines if the data from the specified disk space is resident in the cache store includes a segment descriptor table. The segment descriptor table includes an entry corresponding to each segment in the cache store. Each entry contains various information regarding the segment such as the disk to which it must be returned, the space on the disk to which it must be returned, various indicators indicating such information as whether or not the segment has been written to but not returned to its disk space since being written to, and so forth. Segments in the cache store that have been modified since they were last brought from disk space are returned to their disk space under the control of the segment descriptor table. This transfer takes place as described in application Ser. No. 207,097, when the storage control unit has no other more important task to perform.
The segment descriptor table is stored in a random access memory which is completely separate from the cache store. Should the memory storing the segment descriptor table fail then, in the absence of the present invention, there would be no way for the system described in application Ser. No. 207,152 to access the data segments in the cache store and return them to their appropriate disk space.